Evolved radio access schemes and wireless networks for cellular mobile communication (hereinafter, referred to as “Long Term Evolution (LTE)” or “Evolved Universal Terrestrial Radio Access (EUTRA).” have been standardized by 3rd Generation Partnership Project (3GGP). LTE employs orthogonal frequency division multiplexing (OFDM), which is multi-carrier transmission, as a communication scheme for wireless communication from the base station apparatus to the mobile station apparatus (downlink; referred to as DL.). LTE also employs single-carrier frequency division multiple access (SC-FDMA), which is single-carrier transmission, as a communication scheme for wireless communication from the mobile station apparatus to the base station apparatus (uplink; referred to as UL.). Specifically, LTE employs DFT-Spread OFDM (Discrete Fourier Transform-Spread OFDM) as the SC-FDMA scheme.
LTE-A (LTE-Advanced), which is advanced LTE and employs new techniques, is being studied. It is assumed that LTE-A at least supports the same channel structure as that employed in LTE. A channel refers to a medium used for transmission of a signal. Channels used in the physical layer and channels used in the medium access control (MAC) layer are respectively referred to as physical channels and logical channels. Kinds of the physical channels include a physical downlink shared channel (PUSCH) used for transmission and reception of downlink data and control information, a physical downlink control channel (PDCCH) used for transmission and reception of downlink control information, a physical uplink shared channel (PUSCH) used for transmission and reception of uplink data and control information, a physical uplink control channel (PUCCH) used for transmission and reception of control information, a synchronization channel (SCH) used to establish downlink synchronization, a physical random access channel (PRACH) used to establish uplink synchronization, a physical broadcast channel (PBCH) used for transmission of downlink system information, and so on. The mobile station apparatus or the base station apparatus maps signals generated from control information, data, etc. to respective physical channels, and transmits the signals. Data transmitted on the physical downlink shared channel or the physical uplink shared channel is referred to as a transport block.
Control information mapped to the physical uplink control channel is referred to as uplink control information (UCI). The uplink control information is control information (reception confirmation response; ACK/NACK) representing acknowledgement (ACK) or negative acknowledgement (NACK) for data mapped on the received physical downlink shared channel, control information (Scheduling Request: SR) representing a request for uplink resource allocation, or control information (Channel Quality Indicator: CQI) representing downlink reception quality (also referred to as channel quality).
<Coordinated Communication>
Inter-cell coordinated communication (Cooperative Multipoint: CoMP communication) in which communication is performed with coordination among neighboring cells is being studied for LTE-A in order to reduce or suppress interference on a mobile station apparatus in a cell-edge area or in order to increase power of a received signal. For example, a configuration in which the base station apparatus performs communication using one given frequency band is called a “cell”. For example, as inter-cell coordinated communication, a method (also referred to as Joint Processing or Joint Transmission) is being studied in which signal processing (precoding processing) is applied to a signal using different weights for different cells and a plurality of base station apparatuses transmit the resulting signal to a single mobile station apparatus in a coordinated manner. This method can increase a signal-to-interference-plus-noise power ratio at the mobile station apparatus and improve reception performances at the mobile station apparatus. For example, as inter-cell coordinated communication, a method (Coordinated Scheduling: CS) is being studied in which scheduling is performed for a mobile station apparatus with coordination among a plurality of cells. This method can increase a signal-to-interference-plus-noise power ratio at the mobile station apparatus. For example, as inter-cell coordinated communication, a method (Coordinated beamforming: CB) is being studied in which beamforming is applied to a signal with coordination among a plurality of cells and the resulting signal is transmitted to a mobile station apparatus. This method can increase a signal-to-interference-plus-noise power ratio at the mobile station apparatus. For example, as inter-cell coordinated communication, a method (Blanking or Muting) is being studied in which a signal is transmitted using a certain resource in one cell but no signal is transmitted using the certain resource in another cell. This method can increase a signal-to-interference-plus-noise power ratio at the mobile station apparatus.
As for a plurality of cells used for coordinated communication, different cells may be configured by different base station apparatuses, different cells may be configured by different RRHs (Remote Radio Heads which are outdoor radio units smaller than base station apparatuses and also referred to as Remote Radio Units: RRUs) managed by the same base station apparatus, different cells may be configured by a base station apparatus and an RRH managed by the base station apparatus, or different cells may be configured by a base station apparatus and an RRH managed by another base station apparatus different from the base station apparatus.
A base station apparatus having wide coverage is generally referred to as a macro base station apparatus. A base station apparatus having narrow coverage is generally referred to as a pico base station apparatus or femto base station apparatus. In general, applications of RRHs in a coverage area narrower than that of the macro base station apparatus are being studied. A deployment such as a communication system which includes a macro base station apparatus and an RRH and in which coverage supported by the macro base station apparatus includes part or entirety of coverage supported by the RRH is referred to as a heterogeneous network deployment. A method is being studied in which the macro base station apparatus and the RRH transmit a signal to a mobile station apparatus located in their overlapping coverage in a coordinated manner in a communication system of such a heterogeneous network deployment. Here, the RRH is managed by the macro base station apparatus and its transmission and reception are controlled by the macro base station apparatus. In addition, the macro base station apparatus and the RRH are connected to each other via a wired line such as optical fiber or a wireless line based on a relay technology. The macro base station apparatus and the RRH perform coordinated communication using partially or entirely the same radio resources in this way, whereby overall spectral efficiency (transmission capacity) in the coverage area of the macro base station apparatus can be improved.
While being located near the macro base station apparatus or RRH, the mobile station apparatus is able to perform single-cell communication with the macro base station apparatus or RRH. That is, the mobile station apparatus performs communication with the macro base station apparatus or RRH so as to transmit and receive signals without using coordinated communication. For example, the macro base station apparatus receives an uplink signal from the mobile station apparatus located near the macro base station apparatus. For example, the RRH receives an uplink signal from the mobile station apparatus located near the RRH. Further, while being located near the edge (cell edge) of the coverage of the RRH, the mobile station apparatus requires some measures against co-channel interference from the macro base station apparatus. A method is being studied in which a CoMP scheme enabling coordinated operation of neighboring base station apparatuses is used as multi-cell communication (coordinated communication) of the macro base station apparatus and the RRH so as to reduce or suppress interference on the mobile station apparatus in a cell-edge area.
It is also studied that the mobile station apparatus receives a signal transmitted from both the macro base station apparatus and the RRH using coordinated communication on the downlink and transmits a signal in a form suitable for the macro base station apparatus or RRH on the uplink. For example, the mobile station apparatus transmits an uplink signal at a transmit power suitable for the signal to be received at the macro base station apparatus. For example, the mobile station apparatus transmits an uplink signal at a transmit power suitable for the signal to be received at the RRH. In this way, unnecessary uplink interference can be reduced and spectral efficiency can be improved.
For a data signal reception process, the mobile station apparatus needs to obtain control information representing a modulation scheme, a coding rate, the number of spatial multiplexing, a transmit power adjustment value, resource allocation, and so forth used for a data signal. Introduction of a new control channel for transmitting control information regarding a data signal to LTE-A is being considered (NPL 1). For example, how to improve the overall capacity of control channels is being studied. For example, how to support frequency-domain interference coordination for the new control channel is being studied. For example, how to support spatial multiplexing for the new control channel is being studied. For example, how to support beamforming for the new control channel is being studied. For example, how to support diversity for the new control channel is being studied. For example, the use of the new control channel on a new type of carrier is being studied. For example, it is being studied that a reference signal used in common by all mobile station apparatuses located in a cell is not to be transmitted on the new type of carrier. For example, it is being studied that the reference signal used in common by all mobile station apparatuses located in a cell is transmitted less frequently on the new type of carrier than on the currently used carrier. For example, it is being studied that a signal such as control information is demodulated by the mobile station apparatus using a specific reference signal for the new type of carrier.
For example, in order to implement beamforming, employment of coordinated communication and multi-antenna transmission for the new control channel is being studied. Specifically, it is being studied that a plurality of base station apparatuses and a plurality of RRHs supporting LTE-A apply precoding processing to signals of the new control channel and also apply the same precoding processing to a reference signal (RS) used to demodulate the signals of the new control channel. Specifically, it is being studied that a plurality of base station apparatuses and a plurality of RRHs supporting LTE-A transmit the signals of the new control channel and the RS that are subjected to the same precoding processing, after mapping them to a resource region where the PDSCH is mapped in LTE. It is being studied that a mobile station apparatus supporting LTE-A acquires control information by demodulating the signals of the new control channel having been subjected to the precoding processing, by using the received RS having been subjected to the same precoding processing. With this method, the base station apparatus and the mobile station apparatus no longer need to exchange information regarding the precoding processing applied to the signals of the new control channel therebetween.
For example, in order to implement diversity, a method is being studied in which signals of the new control channel are configured using non-contiguous resources in the frequency domain so as to obtain the advantage of frequency diversity. On the other hand, for the case where beamforming is applied to the new control channel, a method is being studied in which the signals of the new control channel are configured using contiguous resources in the frequency domain.
For example, in order to support spatial multiplexing, employment of MU-MIMO (Multi User-Multi Input Multi Output) enabling multiplexing of control channels addressed to different mobile station apparatuses on the same resource is being studied. Specifically, it is studied that the base station apparatus transmits reference signals that are orthogonal between different mobile station apparatuses and also spatially multiplexes and transmits different signals of the new control channels on a common resource. For example, spatial multiplexing of different signals of the new control channels is implemented by applying beamforming (precoding processing) suitable for each of the different signals of the new control channels.